Push-to-talk (PTT) wireless voice communication has a long history, stretching back to the “handy-talky” military radio of World War II and encompassing various forms of “walkie-talkie” military, police/fire, commercial, hobbyist, and even toy radios. Many such systems have relied on peer to peer radio communication, in which each user transmits directly to all other users via a shared radio spectrum. This technique limits range to that of the individual unit and its transmit power. Other systems, particularly including the dedicated trunked-radio systems deployed for commercial users and first responders, employ repeaters with greater transmit power and receive sensitivity than a handheld unit can achieve, thereby multiplying the reach of a network significantly.
Most recently, PTT systems have been built on top of cellular radio network technologies, allowing the specialized group-communication needs of typical PTT users to take advantage of the commercially successful and widely deployed cellular networks. These systems expand upon the repeater topology model of trunked radio, relying on multiple interconnected base stations to extend the reach of the power-limited handsets, rather than the very large repeater stations typical of trunked radio installations. A development of particular note in this area is the application of Mobile Satellite Services (MSS) systems to PTT group communication, embodied in the Distributed Tactical Communication System (DTCS) developed by the US Navy for Marines deployed around the world. As described in http://www.defenseindustrydaily.com/217M-for-Phase-II-of-Netted-Iridium-Program-05483/, this system builds upon the Iridium MSS system to provide voice group communication for up to 2,000 users in an area up to 250 miles wide. Per http://www.iridium.com/DownloadAttachment.aspx?attachmentID=1197, each spot beam of an Iridium satellite is about 250 miles in diameter, suggesting that DTCS service is provided only within the coverage area of a single beam. In addition to the existing capability, according to http://investor.iridium.com/releasedetail.cfm?ReleaseID=556479, enhanced capabilities under development at the time of this writing promise to expand the DTCS user capacity by a factor of 30. Though not stated explicitly in the public information, this would seem to imply an increase in the number of group members to some tens of thousands (2,000×30=60,000), and may also imply an increase in the range of coverage to incorporate multiple beams in the serving area for a particular group. It is possible that techniques taught in U.S. Pat. No. 6,577,848 may be incorporated in both current and future DTCS designs. In any case, a PTT system built on the Iridium MSS system is of special interest due to its global coverage and ability to provide service in areas that do not have trunked radio or cellular coverage.
In all this prior art, the primary goals of each PTT system have been to adjudicate which user is allowed to speak, and to relay that user's speech burst to all the other users. The older systems relied upon the people themselves to resolve channel utilization conflicts using social protocols, also known as “floor control,” and used direct frequency modulation techniques to convey the analog audio over the radio. Current systems employ digital packet protocols to negotiate floor control, digital speech coding with compression to represent the talker's speech burst (also known as an “utterance”), and digital radio modulation techniques to carry both. Even the latest systems with digital speech coding and the deepest available compression techniques require at least 2,400 bits to convey each second of the talker's utterance over the network, and current designs attempt to match the channel resource to that bit rate so as to convey each utterance in real time. An average utterance lasting 5 seconds thus transmits at least 1,500 bytes over 5 seconds of channel time, plus whatever overhead is required for channel management and floor control in the specific system. Regarding floor control as well, the protocols are generally designed based on the assumption that any group member may attempt to speak at any time; this assumption tends to limit the scale of groups. Outside the domain of PTT service, techniques exist in the prior art that allow users to dictate a message into the Short Message Service (SMS) capability ubiquitous in current cellular handsets, as well as to read back a message received via SMS. This technology is normally used to avoid the use of hands for messaging while driving; it is not normally used to carry on a real-time conversation, for which an ordinary phone call is well suited, and more efficient. However, this speech-to-text and text-to-speech technology is available for application in the PTT domain, and offers the opportunity for extreme compression. For example, the 5-second, 1500-byte utterance mentioned above may require only 150 bytes to convey if transcribed as text.
Another attribute of prior art systems, and particularly the cellular-based digitally-coded systems in common use today, is their use of a power-optimized traffic channel to convey the coded speech of each utterance. These radio channels are typically run at a power level that assumes not quite perfect channel conditions, but that still requires each user in the group to be holding his or her handset in the talking/listening position wherein the antenna is deployed appropriately. In Iridium, for example, it is well known that the traffic channel power level is so low that during a call the handset must either be attached to a separate antenna mounted in an optimal location, or held to the ear with the embedded antenna extended and angled properly above the head. This likely applies in DTCS as well, although no public information is available to confirm it.
The system attributes described in the two foregoing paragraphs support the observation that some conceivable applications of PTT service are not effectively satisfied by existing prior art systems due to scalability constraints linked to the specific efficiency and capacity considerations associated with the usage assumptions cited. In particular, situations in which most users are only listeners and who either do not need to or cannot keep a handset in optimal position are excluded from the groups supportable in the prior art. What is needed, then, is a system that supports such groups, which may number 10,000 to 1,000,000 or more members listening as a background activity but which feature only a handful of potential speakers.
It is thus a principal aim of the present invention to provide a system that supports PTT services for such large and asymmetric groups, optimizing resource utilization differently from the prior art using novel techniques and construction not found in the prior art.